Differential pressure transducer

ABSTRACT

A differential pressure transducer utilizing a transformer and a means for maintaining a constant voltage sum on the secondary leads of the transformer to eliminate errors due to spurious electrical impulses in the transducer. The extensible end of a bellows moves a magnetically permeable core with respect to the transformer, thereby inducing different voltages on separate secondary loops longitudinally spaced along the transformer.

United States Patent Breunich et al.

[4 1 May 16, 1972 [54] DIFFERENTIAL PRESSURE TRANSDUCER [72] Inventors:Theodore R. Breunich, Stamford, Conn.;

Emanuel Dinoia, Briarcliff Manor, NY.

[73] Assignee: Universal Oil Products Company, Des

[56] References Cited UNITED STATES PATENTS 3,555,405 9/1967 Martin..323/51 Primary ExaminerDonald O. Woodie] Attorney-James R. Hoatson,Jr. and Charles H. Thomas, Jr.

[57] ABSTRACT A differential pressure transducer utilizing a transformerand a means for maintaining a constant voltage sum on the secondaryleads of the transformer to eliminate errors due to spurious electricalimpulses in the transducer. The extensible end of a bellows moves amagnetically permeable core with respect to the transformer, therebyinducing difierent voltages on separate secondary loops longitudinallyspaced along the transformer.

6 Claims, 2 Drawing Figures 1967, abandoned.

[52] U.S.Cl. ..73/398R,73/407, 323/51 51 lnt.Cl. ..(;0n9/10 [58] Fieldof Search ..73/398 A, 407 R; 336/30; 323/51 .9 V 5 all 44 PHENTEDHAY 1sm SHEET 1 OF 2 F g u r e l0 /V VE/V 70/? Theodore R. Breun/ch By.Emanuel J. Di/Vo/a A T 7'0 1? NE Y5 DIFFERENTIAL PRESSURE TRANSDUCERThis application is a continuation in part of application Ser. No.819,918 filed Apr. 28, 1969, now abandoned.

This invention relates to a differential pressure transducer utilizing atransformer and a means for maintaining a constant voltage sum on thesecondary leads of the transformer to eliminate errors due to spuriouselectrical impulses in the transducer. The extensible end of a bellowsmoves a magnetically permeable core with respect to the transformer,thereby inducing different voltages on separate secondary loopslongitudinally spaced along the transformer. The differential pressuretransducer is used to make an accurate analogue conversion from apressure difierential, between pressures at environments to be compared,to a voltage differential, between secondary loops of a transformer.Normally, the conversion that is easiest with which to work is one thatis linear, though the differential pressure transducer of this inventionis not limited to such an application. The invention in its preferredform may be used to operate in any fluid medium over any range ofpressures, though its superiority over conventional differentialpressure transducers is most pronounced when it is activated by achanging depth in sea water. Other possible applications includeaeronautical instrumentation and pressure vessel regulation.

One important feature of this invention is the manipulation of thecurrents produced in the secondary loops of the transformer of thisinvention in order to preclude interference in the final output voltagesor currents by spurious electrical impulses in the system. A portion ofthe core and the primary and secondary loops of the transformer of thisinvention are normally only parts of a linearly variable differentialtransformer (LVDT), which requires additional circuitry for completion.This additional circuitry of the LVDT includes a secondary voltagesummation means or secondary current summation means which is used tocontrol and regulate the primary voltage in a closed loop circuit. Thiscircuitry is used to eliminate spurious voltage changes in the secondaryloops due to supply voltage, and temperature or frequency changes and toimprove the accuracy of the voltage differential readings obtained. Thesecondary current summation circuitry is frequently included in anelectrical unit attached to the housing of the transducer of thisinvention.

It is an object of this invention to produce a differential pressuretransducer which will make an accurate analogue conversion of changes inpressure differential between two pressure sources to changes in voltagedifferential between two secondary loops of a transformer.

It is an object to produce a voltage differential of increased accuracyon the signal output leads of the secondary loops while eliminating theeffects of spurious voltage and current changes in the supply voltage aswell as those fluctuations due to temperature and frequency changes.This improved accuracy is obtained by maintaining constant the sum ofthe voltages appearing at the signal output leads. A detector regulatorcompensates for spurious signals which would otherwise tend to changethis voltage sum.

It is a further object to increase the accuracy of the analogueconversion in the preferred form of the invention by allowing onlyminimal movement of the magnetically permeable core even thoughsignificant variations in pressure differential occur between a variablepressure and a reference pressure. The small core movement minimizeserror induced into the transformer due to hysteresis. In its preferredform, the differential pressure transducer will cause a voltagedifferential across the secondary loops of :5 to 7 volts with a coredisplacement of only 10.10 inches, depending on the voltage input to thetransformer.

It is a further object to create a differential pressure transducerhaving a compact design with easy accessibility to component partsshould replacement of parts ever become necessary. In addition, thisinstrument is sturdily built to withstand rugged treatment over anincreased useful life, as compared with differential pressuretransducers currently in use.

In a broad aspect this invention is in a differential pressuretransducer utilizing a pressure sensing means, a magnetically permeablecore means movable along a linear path and operatively connected to saidpressure sensing means, and a transformer axially aligned about saidlinear path and having an axial opening therethrough within which saidmagnetically permeable core means moves and having a primary coil andhaving separate secondary loops spaced longitudinally along said linearpath, the improvement comprising a bridge rectifying means connected tosaid secondary loops and terminating in output signal secondary leads,means for maintaining a constant voltage sum on said secondary leadscomprising a detector-regulator on one side of the input to said primarycoil. resistors in series across said output signal secondary leads, anda feedback lead from between said resistors in series to said detectorregulator.

Preferably, the pressure sensing means, the magnetically permeable core,and the transformer are all located within the same housing. Thisfeature of unitary construction reduces the vulnerability of the unit toshock and other mistreatment. The circuitry for maintaining a constantvoltage sum on the secondary output leads may be located in the samehousing or in an electrical regulating box attached to the main housing.

In the applications of this differential pressure transducer whereaccuracy is required, a non-magnetic metal sheath or guide may be usedto surround at least a portion of the core. This non-magnetic sheath isfastened to the housing and thereby stabilizes the core. Such a sheathis particularly useful where the core is quite long, since the core ismounted as a cantilever onto the extensible end of the bellows and thefree end of the core is relatively unstable. The support furnished tothe free end of the core by the sheath prevents this instability fromintroducing error into the system as voltage fluctuations in thesecondary loops.

The advantages and detailed construction of the foregoing features, asincorporated in the preferred embodiment of this differential pressuretransducer will be made more apparent upon reference to the accompanyingdrawings and the following description thereof.

FIG. 1 is a partially cutaway sectional view of the preferred embodimentof the differential pressure transducer of this invention.

FIG. 2 indicates diagrammatically an electrical circuit of the linearlyvariable differential transformer type working to maintain a constantvoltage sum of the secondary leads.

Referring now to FIG. 1 there is shown a main housing 1 to which anelectrical regulating box 32 is attached by machine screws (not shown).Main housing 1 has several separate cavities located therein and iscomprised of several separate pieces. Housing 1 encompasses a firstcavity 45, within which is located bellows 4. Bellows 4 has a fixed end46 welded to upper bellows base 3, a part of housing 1. Upper bellowsbase 3 is fastened to the balance of housing 1 by means of screws 23.There is an axial passageway 49 which extends through upper bellows base3 and which opens into the interior of bellows 4. Housing 1 also has asecond cavity 50 which is in communication with the interior of bellows4 by means of passageway 49. First cavity 45 and second cavity 50 areseparated from each other by means of an air tight seal formed by O-ring27 which is positioned between upper bellows base 3 and the balance ofhousing 1. The cavities 45 and 50 together with the bellows 4 form thepressure sensing means of this invention.

Pressure ports 44 and 2 are welded to housing 1 at channels in housing 1leading to first cavity 45 and second cavity 50 respectively. Pressureports 44 and 2 are adaptable to pipe fittings, tubes, and otherconnecting devices so that pressure from exterior sources can bechanneled through pressure port 44 to first cavity 45 and throughpressure port 2 to second cavity 50. Pressure within first cavity 45thereby acts against the adjacent walls of housing 1 including the innersurface of removable cover 10, the outer surfaces of bellows 4, thelower end of upper bellows base 3, and pressure port 44. Pressure withinsecond cavity 50 acts upon the adjacent surfaces of housing 1, upon thewalls of passageway 49, upon the inner surfaces of bellows 4, uponclosed extensible end 47 which is welded to lower bellows base 6, uponthe interior surface of core sheath or guide 11, and upon pressure port2. Of the surfaces against which pressures in first cavity 45 and insecond cavity 50 act, only extensible end 47 and the interior andexterior surfaces of bellows 4 are displaceable. Therefore, whenpressure within first cavity 45 increases with respect to pressurewithin second cavity 50, bellows 5 will contract thereby movingextensible end 47 of bellows 4 closer to upper bellows base 3. When thepressure within second cavity 50 increases with respect to pressure infirst cavity 45, extensible end 47 of bellows 4 moves axially away fromupper bellows base 3, thereby expanding bellows 4. v

Housing 1 is equipped with a third cavity 51, within which is locatedtransformer 13. In assembling the pressure differential transducer, coreguide 11 is lowered from the opening in the top of housing 1 into thirdcavity 51. Core guide 11 is threadably engaged to housing 1, locked inplace by locking screws 52, and sealed against housing 1 by means ofO-ring 26, thereby creating a pressure tight seal between second cavity50 and third cavity 51. A transformer adjustment platform having anaxial threaded hole is threadably engaged to a lower externally threadedportion of core guide 11. Platform 15 can thereby be variably positionedwith respect to core guide 11 to adjust the longitudinal positioning oftransformer 13 within cavity 51. Transformer 13 is securely held inplace by upper locking plate 12. Plate 12 has a threaded axial holetherethrough which is engaged around the threaded upper portion of thestem of core guide 11. Upper locking plate 12 is threaded upon the stemof core guide 11 until it is resisted by the upper surface oftransformer 13. The resulting pressure at the thread interfaces betweenupper plate 12 and the stem of core guide 11 and between the threadedinterfaces between platform 15 and core guide 11 thereby locks upperplate 12in place against core guide 1 1 and platform 15 against coreguide 11 and holds transformer 13 rigidly in place. There is anindentation formed in the upper wall of housing 1 which allows the cable37 to pass from transformer 13 up and around upper plate 12 and out ofhousing 1 through a passageway 8 into cavity 51 in the wall ofhousing 1. Cable 37 passes from housing 1 into electrical regulating box32, where there is located the circuitry for maintaining a constantvoltage sum on the secondary leads as illustrated in FIG. 2. Cable 37contains the wires 38 and 39 which lead from terminals A and Brespectively in electrical interface 40 of electrical regulating box 32to the primary coil 31 of transformer 13. Cable 37 also contains wires33, 34, 35, and 36 which connect secondary loops 29 and 29 of thesecondary coil of transformer 13 to bridge rectifier means 48, locatedin electrical regulating box 32.

There is an opening in housing 1 into third cavity 51 at the upper endof housing 1. This opening is covered by a removable cover which issealed to the adjacent edges of the balence of housing 1 by gasket 53.An air tight seal is formed when machine screws 22 are used to fastencover 20 to the remainder of housing 1. Transformer 13 is therebyremovably positioned within the third cavity and is accessible uponremoval of cover plate 20, a part of housing 1, and upper adjustmentplate 12.

At the lower end of housing 1, within cavity 45, a core means 5 is fixedto the extensible end of bellows 4. Core means 5 is comprised of lowerbellows base 6, and extension 24, shaft 54, and magnetically permeableslug l4. Shaft 54 of core means 5 extends from lower bellows base 6axially back through bellows 4, through cavity 50, and into the annularopening within core guide 11 which in turn is positioned within anannular opening of transformer 13. Shaft 54 is threadably engaged tomagnetically preamble slug 14, at the transformer end of core means 5.The movement of the lower end 47 of bellows 4 thereby alters theposition of slug 14 along a linear path with respect to transformer 13.It is this movement which induces a change in voltage in the secondaryloops 29 and 29 of transformer 13. Secondary loops 29 and 29' are spacedlongitudinally along the transformer axis which is also the linear pathof movement of the slug 14 so that slug 14 may be more directly radiallyaligned with one of the secondary loops than with the other. It is thisdifference in alignment which creates different voltages on leads C andD at electric interface 40.

To increase the effectivelife and to increase the possible pressuredifferentials to which the bellows of this unit may be subjected, aspring support may be fastened to the extensible end of the bellows.Below lower bellows base 6 is fixed a spring support 9, secured in placeagainst lower bellows base 6 by means of a stop nut 55 threadablyengaged onto threaded end extension 24 of core 5. Spring support 9 isthereby fastened to the extensible end 47 of bellows 4. At the upper endof spring support 9 a collar 56 flares radially outward from bellows 4and extends a sufficient radial distance to provide a support toaccommodate spring 7. Spring 7 is a square coil spring machined fromsolid stock and having solid ends and preloaded in a compressedcondition. This design eliminates variable end conditions, such ascocking and rotation and makes possible close control of spring rate andlinearity. Spring 7 is compressed between collar 56 of spring support 9and spring adjustment ring 18 and is axially aligned with bellows 4.Spring adjustment ring 18 is threaded on its exterior surface andthereby threadably engaged to the inner threaded walls of housing 1.Spring adjustment ring 18 has an axial hole therethrough so that it maypass over spring support 9 as well as bellows 4 and adjustment nut 55 asit is threaded into housing 1. The hole through ring 18 is ofsufficiently small diameter so that an annular shoulder broad enough tosupport spring 7 remains at the perimeter of ring 18. The spring andbellows are chosen and the spring is preloaded so that the maximumpossible deflection of the expansible end of the bellows is within therange at which the relationship of the bellows load to axial deflectionof the spring is most linear. The spring can either coaxially surroundthe bellows or it may have a common axis with the bellows butlongitudinally displaced from the bellows and adjacent to the extensibleend of the bellows.

Through the use of spring 7, collar 56 and adjustment ring 18, a changein voltage differential between the secondary loops 29 and 29' oftransformer 13 resulting from a relatively large change in pressuredifferential between the pressures in cavities 45 and 50 can be attainedwith only a very minimal movement of bellows 4. Through adjustment ofthe axial positioning of transformer 13 and adjustment of the preload onspring 7, a null point can be achieved whereby voltages are equal atleads C and D for a specific pressure differential between cavities 45and 50. For example, spring 7 is compressed, by adjusting adjustmentring 18, so that it offsets the median pressure differential likely tooccur between second cavity 50 and first cavity 45. The position oftransformer 13 is longitudinally adjusted so that the voltage at Cequals the voltage at D at the electrical interface for this medianpressure differential. If second cavity 50 is the high pressure chamberand first cavity 45 is the reference chamber, an increase in thepressure differential between cavity 50 and cavity 45 will cause thelower end 47 of bellows 4 to move axially downward, thereby furthercompressing spring 7. This will cause slug 14 to move out of directaxial alignment with secondary loop 29 while remaining in direct axialalignment with secondary loop 29. This results in a greater voltage at Cwith respect to the voltage at D because of the larger inductioneffected in secondary loop 29' due to the magnetically permeable slug14. Conversely, a decrease in pressure difierential between cavity 50and cavity 45 will cause the compressive force in spring 7 to overcomethe force resulting from the pressure differential between cavity 50 andcavity 45, and thereby force bellows 4 to contract. This allows spring 7to expand. However, the linear displacement of end 47 of bellows 4 isonly a fraction of what it would be were bellows 4 allowed to expand andcontract freely without the inhibiting influence of spring 7. Thismovement is sufficient however, to cause slug 14 to move out of axialalignment with secondary loop 29 while remaining in direct axialalignment with secondary loop 29. This results in a greater voltage at Dwith respect to the voltage at C.

Because of the minimal movement of the extensible end 47 of bellows 4which is sufficient to create a variation in voltage differentialbetween the secondary loops of transformer 13, an over travel stopadjusting nut 19 is threaded into housing 1 behind ring 18. This overtravel stop adjusting nut 19 prevents damage to the instrument bylimiting the expansion of the bellows 4 by stopping the longitudinalmovement of that portion of spring support 9 adjacent to the extensibleend 47 of bellows 4. Similarly, the over travel stop adjusting nut 19limits the contracting movement of bellows 4 by halting stop nut 55 inits longitudinal movement toward fixed end 46 of bellows 4.

There is an opening into the first cavity 45 of housing 1 at thethreaded end of housing 1. An O-ring 25 is positioned at the edges ofthe threaded end of the balance of housing 1 and a removable cover 10, apart of housing 1, is threadably fastened across the opening, therebytrapping O-ring 25 which forms an air tight seal between cover 10 andthe remainder of housing 1. Removable cover 10 thereby protects spring7, spring support 9, core means 5, and bellows 4, while still allowingthese component parts to be removed from housing 1.

In the electrical regulating box 32, there is an electrical connectioninterface 40, containing interface terminals A, B, C, and D. Theseterminals are the connections at which exterior power and exteriorinstruments are connected to the LVDT. Parts of the LVDT are common tothe differential pressure transducer of this invention. The LVDT, asused with the illustrated embodiment of this invention, comprises thecircuitry contained in electrical regulating box 32 for maintaining aconstant voltage sum on the secondary leads C and D as well as theprimary coil 31 and the secondary loops 29 and 29 of the transformer 13.The entire circuitry of the LVDT is illustrated in FIG. 2.

As illustrated in FIG. 2, an alternating current voltage input at A andB induces a fluctuating magnetic field about the primary coil 31 of theLVDT. This fluctuating magnetic field induces separate alternatingcurrents in secondary loops or windings 29 and 29. Loops 29 and 29 areeach comprised of a series of windings about the primary coil 31. Loops29 and 29 are separate and are longitudinally displaced from each otherwith respect to the axis of the transformer 13. Secondary loop 29 isnearest locking plate 12 while secondary loop 29' is nearest transformeradjustment platform 15. The magnitude of the electric current induced ineach of loops 29 and 29' is governed by the proximity of slug 14 to theprimary coil and to the secondary loop concerned. A movement of slug 14of core means 5 will change the voltage output from the secondary loops29 and 29. As shown diagrammatically, the secondary loop outputs areconnected to full wave bridge rectifiers 48 in electrical regulating box32 by wires 33, 34, 35, and 36. Bridge rectifiers 48 which are connectedin opposition to each other and are grounded at their point ofconnection, and are in turn connected to suitable resistors such as 41,42, and 43, to provide a desired direct voltage on output signalsecondary leads 16 and 17 which in turn provide the varying directcurrent voltage output levels at C and D. The actual resulting outputinduced by the core slug 14 in the LVDT unit and as modified by thedemodulating circuit is thereby transmitted to terminals C and D inelectrical interface 40.

In the LVDT utilized in this invention, the sum of the voltages at C andD, that is the total voltage output with respect to ground of thevoltages at C and D, is always constant throughout the operation of thedifferential pressure transducer of this invention. The stability ofthis total output is a measure of the spurious electrical impulses whichare induced into the system through hysteresis, variations intemperature, and other undesireable sources. This constant voltage sumserves as the means by which such spurious signals can be eliminatedfrom the system. The voltage sum is maintained constant by the secondarycurrent summation circuitry, which is comprised of resistors 57 and 58,feedback wire 28, and detector-regulator 30 placed between segments ofwire 39 on one side of the input to the primary coil 31. Thesignificance of this circuitry can be more fully explained by anexample. in the null or neutral condition, the slug 14 is symmetricallypositioned with respect to the secondary loops 29 and 29. Due to the ACcurrent in primary coil 31, the voltages appearing at the output signalsecondary leads of the two bridge rectifiers are equal but opposed toeach other. For example, if the voltage from the upper regulator 48 is+30 volts, then the voltage from the lower rectifier 48 will be 30volts. The total potential differential, therefore, is an absolute valueof 60 volts. lf the resistors 57 and 58 have equal resistance values,(though they need not have), the voltage appearing on the feedback wire28 would then have an absolute value of 30 volts. That is, its voltagewould be 30 volts above the lowerrectifier output voltage and 30 voltsbelow the upper rectifier output voltage. This condition at feedbackwire 28 is the reference condition in detector-regulator 30. Deviationsfrom this reference condition will cause counteracting adjustments inthe voltage in primary coil 31 by activation of detector-regulator 30.

The normal voltage differential occuring between secondary coils 29 and29 due to pressure differential changes will not cause a deviation inthe reference circuit condition. For example, if slug 14 movesvertically upward, and the voltage output from upper rectifier 48increases to +35 volts, the voltage from lower rectifier 48 should andwill change to 25 volts. The voltage differential across the resistor 57and 58 is still an absolute value of 60 volts, however, and the voltagelevel on feedback wire 28 will therefore still be 30 volts above thevoltage level of lower rectifier 48 and 30 volts below the voltage levelof upper rectifier 48. Activation of detector-regulator 30 occurs onlywhen there is a deviation in the sum of the absolute values of thesecondary loop output voltages, as may occur due to spurious electricalimpulses.

In the overall operation of the pressure transducer of this invention,either cavity 45 or cavity 50 can be used as a reference pressurechamber. Assuming for the purposes of illustration that cavity 45 is thereference pressure chamber, cavity 50 is then connected to a highpressure source through outlet port 2. The higher pressure in cavity 50is transmitted to the interior of bellows 4 through passageway 49 inupper bellows base 3. As this pressure base increases, it forces bellows4 to expand. Since bellows 4 is fixed to housing 1 through upper base 3at a fixed end 46 of the bellows, the expansion of bellows 4 takes placealong the bellows axis at extensible end 47 of bellows 4. This expansionmust further compress an already precompressed spring 7, within cavity45, thereby requiring a great increase in pressure with cavity 50 toeffect a minimal movement of extensible end 47 of bellows 4. Thisminimal movement of bellows 4 is transmitted to lower bellows base 6,shaft 54, and then to slug 14, which are all component parts of coremeans 5. The movement of slug 14, though minimal, is sufficient tocreate the desired voltage differential in response to the change inpressure differential between cavity 50 and cavity 45. This movement ofslug 14 away from the vicinity of secondary loop 29 and toward loop 29causes the voltage induced in loop 29 of the secondary coil to decreaseand the voltage induced in loop 29' to increase. The voltage at Cthereby increases with respect to the voltage at D.

When the pressure within cavity 50 decreases with respect to thepressure within cavity 45, the bellows 4 contracts and the extensibleend 47 of bellows 4 moves toward the fixed end 46 of bellows 4, carryingthe core means 5. The movement of slug 14 of core means 5 with respectto the transformer 13 causes the voltage induced in secondary loop 29 toincrease and the voltage induced in secondary loop 29 to decrease. Thisresults in an increased voltage at D and a decreased voltage at C.Throughout the operation of the pressure differential transducer,however, the sum of the voltages at D and C is constant.

In the operation of the differential pressure transducer of thisinvention, either the first or the second cavity may be used as areference while the remaining of these cavities is used to transmit anindependently varying pressure. Therefore, cavity 50 may be used as thereference cavity and cavity 45 may be utilized as the high pressurecavity, as an alternative to the reverse arrangement previouslydiscussed. In such a case, bellows 4 would expand when pressure 45decreased with respect to the pressure in cavity 50, and the bellowswould contract when the pressure within cavity 45 increased with respectto the pressure within cavity 50.

The foregoing detailed description and illustrations of the preferredembodiment of this invention have been given for cleamess ofunderstanding only, and no unnecessary limitations should be construedtherefrom as modifications will be obvious to those skilled in the art.

We claim as our invention:

1. A differential pressure transducer comprising:

a. a housing having first and second separate cavities therein,

b. separate pressure ports connecting each of said cavities to theexterior of said housing,

c. a bellows having a fixed end and an extensible end located in saidfirst cavity and communicating with said second cavity and connected tosaid housing at said fixed end and connected to a core means at saidextensible end,

d. a transformer axially aligned with said bellows and fastened to saidhousing and having an axial opening therethrough and having a primarycoil positioned coaxially about said transformer axis and havingseparate secondary loops coaxial with respect to said primary coil andspaced longitudinally along the transformer axis,

e. bridge rectifying means connected to said secondary loops andterminating in output signal secondary leads,

. means for maintaining a constant voltage sum on said secondary leadscomprising a detector-regulator on one side of the input to said primarycoil, resistors in series across said output signal secondary leads, anda feedback lead from between said resistors in series to saiddetectorregulator, and

g. a magnetically permeable core means fastened to said extensible endof said bellows and extending into said opening in said transformer andlongitudinally positionable with respect to said opening in saidtransformer, whereby a change in pressure differential between saidfirst and second cavities causes a change in voltages to occur in saidsecondary loops of said transformer when voltage is applied to saidprimary coil of said transformer.

2. The differential pressure transducer of claim 1 further characterizedin that said bellows is provided with a spring loading means havingexternal adjustment means thereby to permit adjustment of the voltagedifferential between said secondary loops by minimal movement of saidbellows.

3. The differential pressure transducer of claim 2 further characterizedin that said spring loading means comprises a square wire springpreloaded in a compressed condition.

4. The differential pressure transducer of claim 1 further characterizedin that said resistors in series have equal resistance values.

5. The differential pressure transducer of claim 1 wherein said bridgerectifying means comprises two full wave rectifiers connected inopposition to each other and grounded at their point of connection, andeach of said full wave rectifiers is connected to only one of saidseparate secondary loops.

6. In a differential pressure transducer utilizing a pressure sensingmeans, a magnetically permeable core means movable along a linear pathand operatively connected to said pressure sensing means, and atransformer axially aligned about said linear path and having an axialopening therethrough within which said magnetically permeable core meansmoves and having a primary coil and having separate secondary loopsspaced longitudinally along said linear path, the improvement comprisinga bridge rectifying means connected to said secondary loops andterminating in output signal secondary leads,

means for maintaining a constant voltage sum on said secondary leadscomprising a detector-regulator on one side of the input to said primarycoil, resistors in series across said output signal secondary leads, anda feedback lead from between said resistors in series to said detectorregulator.

* IF i

1. A differential pressure transducer comprising: a. a housing having first and second separate cavities therein, b. separate pressure ports connecting each of said cavities to the exterior of said housing, c. a bellows having a fixed end and an extensible end located in said first cavity and communicating with said second cavity and connected to said housing at said fixed end and connected to a core means at said extensible end, d. a transformer axially aligned with said bellows and fastened to said housing and having an axial opening therethrough and having a primary coil positioned coaxially about said transformer axis and having separate secondary loops coaxial with respect to said primary coil and spaced longitudinally along the transformer axis, e. bridge rectifying means connected to said secondary loops and terminating in output signal secondary leads, f. means for maintaining a constant voltage sum on said secondary leads comprising a detector-regulator on one side of the input to said primary coil, resistors in series across said output signal secondary leads, and a feedback lead from between said resistors in series to said detector-regulator, and g. a magnetically permeable core means fastened to said extensible end of said bellows and extending into said opening in said transformer and longitudinally positionable with respect to said opening in said transformer, whereby a change in pressure differential between said first and second cavities causes a change in voltages to occur in said secondary loops of said transformer when voltage is applied to said primary coil of said transformer.
 2. The differential pressure transducer of claim 1 further characterized in that said bellows is provided with a spring loading means having external adjustment means thereby to permit adjustment of the voltage differential between said secondary loops by minimal movement of said bellows.
 3. The differential pressure transducer of claim 2 further characterized in that said spring loading means comprises a square wire spring preloaded in a compressed condition.
 4. The differential pressure transducer of claim 1 further characterized in that said resistors in series have equal resistance values.
 5. The differential pressure transducer of claim 1 wherein said bridge rectifying means comprises two full wave rectifiers connected in opposition to each other and grounded at their point of connection, and each of said full wave rectifiers is connected to only one of said separate secondary loops.
 6. In a differential pressure transducer utilizing a pressure sensing means, a magnetically permeable core means movable along a linear path and operatively connected to said pressure sensing means, and a transformer axially aligned about said linear path and having an axial opening therethrough within which said magnetically permeable core means moves and having a primary coil and having separate secondary loops spaced longitudinally along said linear path, the improvement comprising a bridge rectifying means connected to said secondary loops and terminating in output signal secondary leads, means for maintaining a constant voltage sum on said secondary leads comprising a detector-regulator on one side of the input to said primary coil, resistors in series across said output signal secondary leads, and a feedback lead from between said resistors in series to said detector regulator. 